A vapour deposition method for preparing an amorphous lithium-containing oxide or oxynitride compound not containing phosphorous comprises providing a vapour source of each component element of the compound, including at least a source of lithium, a source of oxygen, a source of nitrogen in the case of an oxynitride compound, and a source or sources of one or more glass-forming elements; heating a substrate to substantially 180° C. or above; and co-depositing the component elements from the vapour sources onto the heated substrate wherein the component elements react on the substrate to form the amorphous compound.